1. Field of the Art
The present invention relates to a torque detecting apparatus in an electrically operated power steering system.
2. Related Art Statement
In a commonly known electrically operated power steering system, a first steering shaft connected to a steering wheel is coupled, by an elastic coupling device, to a second steering shaft which is connected through a linkage to a vehicle's wheel to be steered, so that the first and second steering shafts are rotatable relative to each other. The second steering shaft is driven by suitable electric drive means such as a servo motor, in response to an electric signal produced by a rotation detector which senses a relative angular displacement of the first and second steering shafts that corresponds to a steering torque applied to the steering wheel upon rotation of the steering wheel.
For detecting an angular displacement of the first steering shaft relative to the second steering shaft, it is generally practiced to convert that relative angular displacement into a linear movement by utilizing an externally threaded screw and a nut engaging the screw, and by sensing the linear movement by a potentiometer. An example of such detector is disclosed in Japanese Patent Application which was laid open in 1983 under Publication No. 58-53562. In the disclosed detector arrangement, an externally threaded shaft is secured to the first steering shaft, so that the threaded shaft engages an internal thread formed in a coupling box fixed to the second steering shaft. The first steering shaft and the threaded shaft are supported so that they are movable in the axial directions when the first steering shaft is rotated relative to the second steering shaft. Upward and downward axial movements of the threaded shaft are limited by an upper and a lower stop member provided within the gearbox. The upper and lower stop members are adapted to abut on an abutment member secured to the end of the threaded shaft. The threaded shaft is normally held in its neutral position by a pair of return springs which act on the threaded shaft in opposite axial directions. In this arrangement, a rotary movement of the first steering shaft is converted into its axial movement by means of the externally threaded shaft engaging the internally threaded coupling box fixed to the second steering shaft. The axial displacement of the first steering shaft is detected by a suitable sensor.